Microcolumn separation techniques, in particular capillary electrophoresis has become a very interesting separation technique which is used as part of a sensor or a chemical analysis system. One major reason for this is the great efficiency of the method as a separation technique. The sampling methods usually applied in capillary electrophoresis are: injection of a sample with a syringe, via a septum, in an injection block, the use of injection valves with/without a sample loop, and dipping one end of the capillary tube into the sample reservoir, whereby the sample is introduced by gravity flow, by over- or underpressure, or by electroendosmosis and/or electromigration.
While it is mentioned in Journal of Chromatography, 452, (1988) 612-622, that sample valves are the most suitable sampling method for capillary electrophoresis, there also is described a valveless device for the injection of a sample. The described arrangement comprises a cast capillary block which is connected between an electrode compartment and a sampling device. In the electrode compartment electrolyte solutions contact electrodes. The capillary tube contains measuring electrodes which are connected with an evaluation electronics. The sampling device consists of a broadened part of the capillary tube connected with two feeders which extend perpendicular to the capillary tube. The arrangement of the two feeders off-set from each other along the longitudinal extension of the capillary tube is such, that the sampling device has the shape of a capillary double T structure.
The sample is introduced into the sampling device via a syringe. The injection volume is defined geometrically by the distance which the two feeders are spaced apart along the capillary tube. The transport of the electrolyte solution and the sample in the capillary tube is accomplished by electric fields that are applied between the respective electrodes along the capillary tube. An advantage of the double T shape sampling device, as is also obtained with the use of injection valves, is the concentration effect of dilute sample ionic species. However, it is possible that, although no electric field gradient over the feeders exists, sample components from the feeders may diffuse into the capillary tube when the sample has already left the sampling position. The amounts of sample components that uncontrollably enter the capillary tube depend on the diffusion coefficients and the mobilities of the respective sample components. Thus, at the detector there not only arrives a more or less broadened plug of injected sample fluid, depending on the diffusion coefficients and the mobilities of the respective components in the electrolyte and the electric field, but also the electrolyte in front and after or between individual plugs of sample fluid is “polluted” with unpredictable amounts of sample components. These unpredictable amounts of sample components reaching the detector are highly undesirable and result in a high noise of the detected signal, thus reducing the limits of detection considerably.
In Analytical Chemistry, 1992, 64, pages 1926 -1932 a capillary electrophoretic device is described in which the sample is injected electrokinetically dipping one end of a capillary into the sample reservoir and applying a voltage across the ends of the capillary. In the electric field the sample is transported electrokinetically and is injected at a T-junction into the channel system of the capillary electrophoretic device. This method, however, leads to a well-known bias of the actual sample composition due to the differences in the electrophoretic mobilities of the sample components. Thus, the sample introduced often does not have the same composition as the original sample. In addition, the volume of the introduced sample is very often unknown such, that internal standards have to be used for quantitative analyses.